1. Field of the Invention
The present invention relates generally to a system and method for controlling an idling speed for an internal combustion engine.
2. Description of the Background Art
A Japanese Utility Model Registration Application First Publication No. Showa 60-188840 published on Dec. 14, 1985 and a U.S. Pat. No. 5,121,725 issued on Jun. 16, 1992 exemplify previously proposed idling speed controlling systems in which an idling control valve is disposed in an auxiliary air passage bypassing a throttle valve so as to adjust an auxiliary air quantity, thus controlling the idling speed to a target engine revolution speed.
A stepping motor has been used to drive the above-described idling control valve. In this case, a pulse train signal whose number of pulses and phase are determined on the basis of the engine revolution speed is supplied to the stepping motor, thereby adjusting an opening angle of the idling control valve.
The number of pulses supplied to the idling control valve is determined according to a control value ISC.sub.on calculated using the following equation (1): EQU ISC.sub.on =ISC.sub.tw +ISC.sub.fb ( 1)
wherein ISC.sub.on denotes a basic control value determined depending on a coolant temperature (hereinafter, referred to as a water temperature) and ISC.sub.on denotes a feedback correction value.
For a feedback control of the idling speed, a comparison is made between, e.g., an actual revolution speed detected by a crank angle sensor and a target revolution speed which is dependent on the water temperature detected by the water temperature sensor. If there is a difference therebetween, the idling speed is controlled to provide the target revolution speed in which the feedback correction value ISC.sub.on is added to the instantaneous control value ISC.sub.on.
In addition, the feedback correction value ISC.sub.fb is set according to the result of proportional-integral (PI) control. To increase a speed responsive characteristic, the differential portion (D) is added which is based on the change speed in the actual revolution speed so as to be set according to the proportional-integration-differential (PID) control.
However, in a case where the stepping motor is used which provides a slower responsive characteristic for the idling control valve than in the previously proposed idling speed control system, the feedback correction value ISC.sub.fb is set according to the proportional-integral-differential (PID) control so as to carry out the feedback control. During an interval at which the differential portion is added, however, the actual revolution speed does not change so largely that an effect of adding the differential portion to the set feedback correction value cannot sufficiently be achieved.